Chemistry Reference
In-Depth Information
actually the fi rst protein types detected to be conjugates with sugars in 1865 ('…
da ß
das Mucin einen gepaarten Stoff darstelle'
[that the mucin may represent a conju-
gated compound]) [14, p. 206], are potent lectin binders exploitable in one-step
purifi cation of lectins (please see Chapter 18.3). Initially, affi nity chromatography
for lectins used cross- linked dextran (Sephadex
®
) as a matrix for the isolation of
concanavalin A [15]. The enormous potential of this strategy was not immediately
realized. A reviewer judged the report after its initial submission to represent 'a
modest advance in an obscure area' [16]. In effect, this technical advance was
instrumental in markedly increasing the publication activity in this fi eld, yielding
a surge in the number of papers on new lectins and applications [17]. The weak
reactivity to cross-linked agarose of the Charcot-Leyden crystal protein is of interest
in this respect, because it indicates reactivity for sugars of this protein, whose
typical hexagonal bipyramidal crystals were fi rst described in the post- mortem
spleen of a leukemia patient in 1853 [18] and in sputum of asthmatics in 1872
[19] . Autocrystallization
in situ
of this protein, which constitutes 7-10% of the
protein content of a mature blood eosinophil (about 8.5pg/cell), may thus be
considered as a physiological purifi cation step.
The technical breakthrough of establishing affi nity chromatography in lectin
research also opened the door for the fi rst experimental purifi cation of a mam-
malian lectin (Table 15.1) [20]. How its presence and functional signifi cance was
traced is recounted in the next section.
15.4
Recent Developments
This fi rst purifi cation of a mammalian lectin was part of a line of research intended
to elucidate the role of ceruloplasmin in maintaining the copper level and details
on the metabolism of this transport protein for copper ions. As a reagent to address
these issues, a radioactive form of the glycoprotein was produced by tritiation. This
reaction required desialylation of
N
-glycan chains (for structural details on these
chains, please see Chapter 6) and oxidation of galactose at its C6 atom, catalyzed
by galactose oxidase [21]. Amazingly, the performed engineering of the
N
- glycan
chains, which unmasked galactose residues, was not without consequences. It
dramatically altered ceruloplasmin's serum clearance: “evidence is presented to
show that, in contradistinction to homologous, native ceruloplasmin, which sur-
vives for days in the plasma of rabbits, intravenously injected asialoceruloplasmin
disappears from the circulation within minutes and accumulates simultaneously
in the parenchymal cells of the liver. The rapidity of this transfer of asialocerulo-
plasmin from plasma to liver has been shown to be dependent upon the integrity
of the exposed, terminal galactosyl residues” [22] . Thus, desialylation turned
N
-
glycans into ligands and, fortunately, the enzymatic oxidation did not impair the
bioactivity for the endocytic hepatic C-type lectin (for further information on C- type
lectins, please see Chapters 16, 19, 20 and 27). It became a role model for targeted
drug delivery by (neo)glycoproteins [23] (for conjugation chemistry in neoglyco-
